Vasopressin (antidiuretic hormone, ADH) and the renin-angiotensin-aldosterone system are major factors involved in the maintenance of the water and electrolyte composition of extracellular fluid (ECF). Regulation of the water and electrolyte composition of the ECF in turn is the primary factor in determining blood volume, and ultimately, blood pressure. In addition to its effects on water balance, ADH may have a direct effect on blood pressure as a result of its ability to increase peripheral resistance. These studies, therefore, are designed to examine the role of ADH and the interaction of ADH with the renin-angiotensin system in one-kidney Goldblatt hypertension in the conscious dog and in the rat with hereditary diabetes insipidus. In addition, these studies are examining the possibility that ADH may be the primary factor causing hypertension in another volume dependent form of hypertension, the DOCA-salt rat model of hypertension. These studies will measure plasma levels of ADH, pituitary ADH content and 24 hours urinary ADH excretion by radioimmunoassay. These measurements will be correlated with radioimmunoassay of plasma renin activity and angiotensin II levels.